Downhole tools are known, for example underreaming and stabilisation tools to be used in a bore hole (see for example PCT Application No. WO 2005/124094).
Drilling, underreaming or other tools are often lowered with the hollow piston held fixedly inside the tubular body and then, when the tool is in position, the hollow piston can be activated. A system for locking the piston has already been envisaged which, in a closed position, holds the hollow piston axially in an immobilisation position and which is brought into the release position by electrical control.
However, in general terms, it is more usual to control the activation of the hollow piston hydraulically, by means of a fluid formed by drilling mud. When the latter circulates in the string connecting the tool to the surface, the piston immobilised by the activation device is subjected to a differential pressure that acts on the piston upwards or downwards. The activation device provides a tubular extension extending the piston. The end of this extension is kept fixed in a tubular sleeve by shear pins. A sleeve bears on a ring forming a stop and fixed to the tubular body of the tool. This stop prevents the movement of the piston subjected to a differential pressure and therefore producing a traction force on the tubular sleeve, as long as this pressure remains below a predetermined threshold.
The shear pins are calibrated so that a traction force of the piston greater than the above-mentioned threshold causes a shearing of the pins and releases the hollow piston, which can then slide in the axial cavity in the tubular body of the tool. In the case of an underreaming tool, the piston, which has become driving, then allows the deployment of a cutting arm.
The calibrated value of the shear pins is a compromise between on the one hand the limit of use of the tool in its dormant mode, that is to say when the hollow piston is not activated, and the limit of the maximum pressure acceptable for the tool or available through the drilling pumps.
In order not to limit the working capacities of the tool in dormant mode and to prevent, which is not always possible, risks of accidental activation that might be caused for example by a pressure jolt, the calibre of the shear pins is designed so as to be relatively high. The shearing is caused by a temporary closure of the tubular sleeve by a ball and/or a very significant increase in the pressure of the drilling mud. The substantial energy stored by the driving piston must then be dissipated on the release of the piston caused by the rupture of the pins. The shock that results from this may cause deformation of parts and impair the subsequent functioning of the tool.
Moreover, experience has shown that dead volumes and play between parts could present an obstacle to the correct function of the mechanism of the tool. This is because the particles present in drilling mud may compact or cement at these places so as to interfere with or prevent the movement of the parts.